How to make a perfect orbit?
Add a circle or any other object (could be an empty) to the position of the sun and parent the planet to it. If you rotate the center object, the planet will rotate too, you can then animate its rotation via the center object's rotation (or just let it rotate on its local axis via Motion Actuator). Its important to create a new center object for every other planet so that their rotations can be different.

Object Relations:


Sun
Orbit Object


Planet (Origin at sun)



Example (Circles used as orbit objects for demonstration):